Actually.. at least some of the time, what happens is a ram has a paint pad strapped to his belly, then he's released into a field of ewes.
The farmer knows which ewes are pregnant by the fact they have bright paint spots on their back.
@potatoe4life I believe you're referring to Tartock's comment about Finland in WWII. Don't remember which comic it was on, but I have a screen cap of it, so I can confirm it has (or had) 111 up-votes.
The farmer knows which ewes are pregnant by the fact they have bright paint spots on their back.